(A) Field of the Invention
This invention relates to the preparation of meta sulfobenzoic acid (m-sulfobenzoic acid) and meta hydroxybenzoic acid (m-hydroxybenzoic acid) therefrom and more particularly relates to a process for the isolation of m-sulfobenzoic acid from a sulfonation reaction mixture.
(B) History of the Prior Art
It has been known in the prior art that m-sulfobenzoic acid can be prepared by sulfonation of benzoic acid with fuming sulfuric acid or sulfur trioxide (see e.g., U.S. Pat. No. 3,094,558; Justus Liebig's Annalen Der Chemie, Vol. 280, pg. 6, 1894 by Kekule et al.; Journal of the American Chemical Society, 1932, beginning at pg. 2009 and "Sulfonation and Sulfation with Sulfur Trioxide", Gilbert et al., Industrial Engineering Chemistry, (1953), pp. 2065-2072.
In such prior art preparations of m-sulfobenzoic acid, the reaction mass was generally dissolved in cold water, usually filtered and the sulfobenzoic acid was salted out with a high sodium chloride salt concentration to precipitate the benzene sulfonate. The crude sulfobenzoic acid was then frequently washed with saturated salt solution which was sometimes cold. The product was then pressed to remove excess water containing the sodium chloride.
Such processes for isolating the m-sulfobenzoic acid were very complicated, often resulted in loss of yield and resulted in products which had a high sodium chloride salt contamination. It was, however, believed that the high sodium chloride concentrations were essential both for salting out and washing purposes in order to prevent large losses of product by means of its solubility in the wash liquid. The resulting m-sulfobenzoic acid products contained high sodium chloride and sometimes contained substantial residual sulfates or bi-sulfates which seem to interfere with the preparation of high quality m-hydroxybenzoic acid from the m-sulfobenzoic acid by fusion of the m-sulfobenzoic acid with alkali metal hydroxide.